Add UMA metric for tracking listeners for blocking touch while fling is happening

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index fb9f9cad7fc391e4195af4b0cf5172ffd2b80c46..8c742f3f259398df50c624afe8f98ea6901f32d0 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -775,6 +775,7 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureFlingStart(
                            TRACE_EVENT_SCOPE_THREAD);
       gesture_scroll_on_impl_thread_ = false;
       fling_may_be_active_on_main_thread_ = true;
+      client_->DidStartFlinging();
       return DID_NOT_HANDLE;
     }
     case cc::InputHandler::SCROLL_IGNORED: {
@@ -1273,6 +1274,7 @@ bool InputHandlerProxy::TouchpadFlingScroll(
       // the subarea but then is flung "under" the pointer.
       client_->TransferActiveWheelFlingAnimation(fling_parameters_);
       fling_may_be_active_on_main_thread_ = true;
+      client_->DidStartFlinging();
       CancelCurrentFlingWithoutNotifyingClient();
       break;
   }